In English, tenses are formed by conjugating an auxiliary verb plus a standard form of the principal verb (the base form, the gerund form, or the past participle form). It is import to focus on the variations in the auxiliary verb to properly use English tenses.
Symbols Used:
S (subject)
Aux (auxiliary verb)
O (objects)
? (question word, i.e., who, what when, etc.)
Construction:
In general, using the following patterns to construct sentences in active sentences.
Positive: S + Verb + O
Negative:S + Aux + Verb + O
Question:(?)+ Aux + S + Verb + (O)
Present Simple
Use the present simple to express regular routines and habits. The present simple is often used with adverbs of frequency.
Auxiliary: DO
Examples:
He often arrives late for work.
When do you do your homework?
They don't take the bus to work.
Present Continuous
Use the present continuous when something is happening at or around the present moment in time. The present continuous is often used with 'now, at the moment, currently, today'.
Auxiliary: BE
Examples:
Jack is working on the report at the moment.
What are you doing?
Go ahead and take it. She isn't reading that book currently.
Present Perfect
Use the present perfect for:
- Something which has happened up to the present moment
- Something which has happened at an unspecified point of time in the past
- Something which has recently happened
Examples:
They haven't done their work yet.
He's worked at this company since 1978.
Have you ever been to Rome?
Present Perfect Continuous
Use the present perfect continuous to express the duration of an activity that begins in the past and continues up to the present moment. In many cases BOTH the present perfect and present perfect continuous can be used.
Auxiliary: HAVE BEEN
Examples:
We've been driving for three hours.
How long have you been sitting at that table?
They haven't been working here long.
Source: http://esl.about.com/od/grammarstructures/a/a_auxpres.htm
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